The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
This disclosure relates to apparatuses and methods for testing fluids, in particular, fluid compositions and methods for well-completion operations during which the fluid compositions are pumped into a wellbore and make contact with tubular bodies and subterranean rock formations.
In the course of completing oil and gas wells and the like, various types of fluids are circulated in the wellbore. These fluids include, but are not limited to, drilling fluids, spacer fluids, cement slurries and gravel-packing fluids. In addition, these fluids typically contain solid particles.
Cement slurries are usually incompatible with most drilling fluids. If the cement slurry and drilling fluid commingle, a highly viscous mass may form that can cause several problems. Cement slurry can channel through the viscous mass. Unacceptably high friction pressures can develop during the cement job. Plugging of the annulus can result in job failure. In all of these situations, zonal isolation may be compromised, and expensive remedial cementing may be required.
Consequently, intermediate fluids called preflushes are often pumped as buffers to prevent contact between cement slurries and drilling fluids. Preflushes can be chemical washes that contain no solids or spacer fluids that contain solids and can be mixed at various densities.
Spacers are preflushes with carefully designed densities and rheological properties. Spacers are more complicated chemically than washes. Viscosifiers are necessary to suspend the solids and control the rheological properties, and usually comprise water-soluble polymers, clays or both. Other chemical components include dispersants, fluid-loss control agents, weighting agents, antifoam agents and surfactants. A thorough discussion concerning the uses and compositions of preflushes may be found in the following publication. Daccord G, Guillot D and Nilsson F: “Mud Removal,” in Nelson E B and Guillot D (eds.): Well Cementing—2nd Edition, Houston: Schlumberger (2006) 183-187.
For optimal fluid displacement, the density of a spacer fluid should usually be higher than that of the drilling fluid and lower than that of the cement slurry. Furthermore, the viscosity of the spacer fluid is usually designed to be higher than the drilling fluid and lower than the cement slurry. The spacer fluid must remain stable throughout the cementing process (i.e., no free-fluid development and no sedimentation of solids). In addition, it may be necessary to control the fluid-loss rate.
Another important function of preflushes is to leave the casing and formation surfaces water wet, thereby promoting optimal bonding with the cement. Achieving water-wet surfaces may be challenging, especially when the drilling fluid has been non-aqueous. Such non-aqueous fluids (NAF) may be oil-base muds, synthetic muds or emulsion muds whose external phase is oil-base. For these circumstances, special dispersant and surfactant systems have been developed by the industry. Designing a dispersant/surfactant system for a particular well may be complicated because several parameters must be considered, including the base oil of the NAF, the presence of emulsifiers, the fluid density, bottomhole temperature, presence of brine salts and the chemical nature of the cement system.
Laboratory tests may be performed to determine the ability of dispersants and surfactants to properly remove NAF from the annulus and leave casing surfaces water wet. The most common methods are “grid tests” and “rotor tests.” Grids are made from screens with different mesh sizes. Rotors are usually steel cylinders whose surfaces may be smooth, rusty, sandblasted to various degrees of roughness, or covered with a screen. The grid or rotor is first immersed in a NAF, the operator verifies that the surfaces are completely coated, and the grid or rotor is weighed. Then the grid or rotor is immersed in an aqueous solution containing dispersants and surfactants at desired concentrations. The grid or rotor may remain stationary or be agitated in the solution for various time periods. Following the immersion period, the grid or rotor is removed and reweighed. The difference between the original and final weight reveals the percentage of NAF removal and the efficiency of the surfactant/dispersant mixture. This method may not be representative of the process that occurs in a well. The test temperature is limited to about 85° C. (185° F.) because it is performed at ambient pressure. In addition, the test does not allow for the use of spacer fluids or other types of fluids that contain suspended solids. When a grid or rotor coated with a solids-laden NAF is immersed in a solids-laden spacer fluid, the grid or rotor may not be solids free upon removal and measuring a weight difference may not provide useful information concerning how well the spacer displaced the NAF.